As it is widely known, the power delivered by an internal-combustion engine depends on the amount of air fed into the combustion chamber of this engine.
Since the amount of air is proportional to its density, the outside air is usually compressed before it is fed into this combustion chamber when high engine power is required.
This operation, referred to as supercharging, can be carried out using any means such as a turbocompressor.
Furthermore, in order to increase even further this amount of air in the combustion chamber of the cylinder, the residual burnt gas initially contained in the dead volume of the combustion chamber is to be discharged in the vicinity of the top dead center of the piston and replaced by supercharged air. This stage is more commonly referred to as burnt gas scavenging and it is generally conducted before the end of the engine exhaust phase.
As it is known from document FR-2,886,342, this scavenging is carried out, at the end of the engine exhaust phase and at the start of the intake phase, when the engine intake pressure is higher than its exhaust pressure. This scavenging stage is performed by overlapping of the exhaust and intake valves of a cylinder by opening simultaneously these valves for some degrees to some ten degrees of crank rotation angle.
The supercharged intake air is thus fed into the combustion chamber before the end of the exhaust phase by expelling the exhaust gas contained therein. This gas is thus discharged through the exhaust valve and replaced by intake air. Document FR-2,926,850 filed by the applicant describes another residual burnt gas scavenging method wherein an exhaust valve opening/closing sequence is carried out during the engine exhaust phase and, during this exhaust valve opening/closing sequence, an intake valve opening/closing sequence is carried out so as to perform residual burnt gas scavenging.
Although this type of engine gives satisfactory results, it however involves quite significant drawbacks.
In fact, for engines running under partial loads, the burnt gas scavenging stage cannot be produced. This is essentially due to the fact that the intake pressure is lower than the exhaust pressure. Therefore, when the exhaust and intake valves are open, the exhaust gas cannot be discharged from the combustion chamber through the exhaust valve.
An amount of residual burnt gas therefore remains in the dead volume and exhaust gas is fed into the intake manifold, thus preventing intake of air into the cylinder.
The present invention aims to overcome the aforementioned drawbacks by means of a burnt gas scavenging method for an engine running under partial loads wherein the exhaust pressure is decreased so as to be lower than the intake pressure.